Metal composites have been known for a long time and are employed in numerous fields. They include, for example, oxide-dispersed metals such as refractory metals provided with yttrium oxide for fine grain stabilization. In other fields, materials of this type based on silver have become established, particularly in the field of contact materials.
U.S. Pat. No. 5,207,842 describes silver-based materials which have been obtained by mixing of silver metal powder with powder composed of tin oxide and tellurium oxide and subsequent pressing and sintering. Materials of this type based on silver and tin oxide are well suited as contact materials.
In Holm Conferences on Electrical Contacts 1982, pages 77-85, Sakairi et al. describe a process for the internal oxidation of silver alloys. For the purposes of the present invention, internal oxidation is a process in which an alloy of silver with lesser noble metals is subjected to oxidizing conditions, which brings about oxidation of the less noble metals but not of silver. Silver-tin oxide materials containing finely divided tin oxide in amounts of from 9.3% by weight to 11.7% by weight and silver-cadmium oxide materials containing finely divided cadmium oxide in amounts of 12.4% by weight were obtained in this way. The materials had to be heat treated in order to make them processable. The internal oxidation was carried out for one week at 450° C. in a pure oxygen atmosphere at a gauge pressure of 9 atmospheres.
U.S. Pat. No. 4,243,413 discloses a silver-tin oxide contact material which contains oxides of tin, nickel and indium. The internal oxidation was carried out for 40 hours at 700° C. under a gauge pressure of 10 atm.
EP-A-508055 discloses silver-based contact materials which can be obtained by internal oxidation of alloys composed of 4-11% by weight of tin, 1-5% by weight of indium, from 0.05 to 4% by weight of tellurium and optionally 0.01-1% by weight of iron, nickel or cobalt and/or from 0.05 to 3% by weight of cadmium, balance silver. The internal oxidation is carried out in an oxidizing atmosphere at from 650° C. to 750° C. for from 8 to 26 hours.
All production processes by means of internal oxidation which have been described are carried out under superatmospheric pressure for a comparatively long time. It would therefore be desirable to make production at a relatively low pressure, for example atmospheric pressure (i.e. without employing superatmospheric pressure), possible. The present invention relates to providing a process for producing silver-based contact materials at a relatively low pressure without significantly impairing the advantageous properties of the materials.